1. Field of the Invention
The present invention relates to a semiconductor laser element which can vary an emission frequency and is suitable for use in, e.g., an optical communication system, and a method of driving the same.
2. Background of the Invention
Semiconductor laser elements of various structures are known. Of these elements, a distributed Bragg-reflection (DBR) variable-wavelength semiconductor laser element has a good monochromatic characteristic of emission light, and can control an oscillation wavelength. For this reason, when the DBR semiconductor laser element is used in optical fiber communications, it can advantageously transmit a large volume of information at a very high speed without being influenced by material dispersion of an optical fiber. Thus, the DBR semiconductor laser has been developed as a future communication light source for coherent optical transmission.
FIG. 1 is a sectional view of a DBR variable-wavelength semiconductor laser element having a conventional structure when viewed from a plane parallel to a resonance direction of the element.
In FIG. 1, an n-type cladding layer 1', an active-optical waveguide layer 2, and a p-type cladding layer 3 are sequentially stacked on an n-type semiconductor substrate 1. In these semiconductor layers, a first DBR portion, an active portion, a phase adjustment portion, and a second DBR portion are juxtaposed in a laser resonance direction. In these portions, electrodes, 4, 5, 6, and 4' are independently formed. A common electrode 7 is formed on the bottom surface of the substrate 1. Grooves 9, 9', and 9" are formed at boundaries of these portions. Diffraction gratings 8 and 8' are formed in the layer 2 of the first and second DBR portions.
In this element, when a current I.sub.LD is injected from the electrode 5 to the active-optical waveguide layer 2, light is emitted. This light propagates through the layer 2, and only light components having specific wavelengths are reflected by the diffraction gratings 8 and 8' in the two DBR portions, thus causing laser resonance. When a current I.sub.tune is injected from the electrodes 4 and 4', a carrier-electron gas is accumulated in the layer 2 by this current. The refractive index of the layer 2 is changed by a plasma effect. When the refractive index of the second layer 2 is changed in this manner, the wavelength of light components reflected by the diffraction gratings 8 and 8' is shifted, so that a laser oscillation wavelength can be changed accordingly. The phase adjustment portion adjusts the phase of light reflected by the DBR portions so as to cause this element to oscillate in a single mode. The phase can be adjusted in such a manner that a current I.sub.phase is injected from the electrode 6, and the refractive index of the layer 2 is changed by utilizing the plasma effect.
Of the two portions in the conventional element, for the DBR portion, since the cycle of the diffraction grating sensed by light is changed due to a change .DELTA.n in refractive index, a Bragg reflection wavelength is shifted. A wavelength fluctuation .DELTA..lambda..sub.b at this time can be expressed as follows by a carrier injection amount .DELTA.N.sub.b which changes according to the current I.sub.tune applied from the electrodes 4 and 4': ##EQU1## where .lambda. is the Bragg wavelength when I.sub.tune =0, n.sub.beff is the effective refractive index of the DBR portion, .xi. is the light confinement coefficient of the layer 2, and .differential.n/.differential.N is the refractive index change coefficient as a function of a carrier change caused by the plasma effect.
In the phase adjustment portion, assuming that a carrier is changed by .DELTA.N.sub.p by the current I.sub.phase applied from the electrode 6, a change .DELTA..lambda..sub.p in oscillation wavelength can be expressed by: ##EQU2## where n.sub.aeff and n.sub.peff are the effective refractive indices of the active portion and the phase adjustment portion, respectively, and l.sub.a, l.sub.p, and l.sub.b are the lengths of the active portion, the phase adjustment portion, and the DBR portion, respectively. Upon comparison between equations (1) and (2), equation (2) can yield a smaller .DELTA..lambda. in unit of the same number of carriers than that of equation (1). More specifically, the current I.sub.phase must be set to be higher than the current I.sub.tune. When a wavelength is to be varied, the element produces heat by the current I.sub.phase, and the refractive index is increased. As a result, a change in width of the refractive index is suppressed, and a wavelength variation width is decreased.
In order to obtain a sufficiently large .DELTA..lambda..sub.p, as can be understood from equation (2), the length of the phase adjustment portion must be increased to some extent. For this reason, a loss is increased, and a threshold current is increased accordingly.
On the other hand, Japanese Patent Laid-Open Nos. 62-2213, 62-241387, 63-133105, and the like disclose techniques for forming a wavelength selective filter using a structure similar to that of the above-mentioned semiconductor laser. FIG. 2 is a side sectional view showing such an example of a conventional wavelength selective filter.
In FIG. 2, a buffer layer 78, an optical waveguide layer 72, an active layer 71, and a cladding layer 79 are sequentially formed on a substrate 77. This multilayered structure is divided into a light gain portion 74, a phase control portion 75, and a DBR portion 76 in a light propagation direction. Electrodes 80.sub.1, 80.sub.2, and 80.sub.3 from which currents can be independently injected are respectively formed on these portions. A diffraction grating 73 is formed in the optical waveguide layer 72 of the DBR portion.
Light incident from one end face of the filter propagates through the optical waveguide layer 72, and emerges from the other end face. In this case, a light component having a wavelength which satisfies a Bragg condition of the diffraction grating 73 is reflected by the DBR portion 76, and does not emerge from the other end face. When a current is injected from the electrode 80.sub.3, a refractive index of the optical waveguide layer 72 is changed by a plasma effect, thus tuning the wavelength of the light component reflected by the diffraction grating 73. When a current is injected from the electrode 80.sub.1, propagation light is given with a gain from the active layer 71. Furthermore, when a current is injected from the electrode 80.sub.2, the phase of the propagation light is adjusted.
In the above-mentioned filter, however, when a current is injected for tuning, a light gain and a naturally discharged light intensity are also changed. For this reason, in order to realize stable wavelength selectivity and to assure sufficient crosstalk with a non-selected wavelength, a carrier injection amount must be limited. The plasma effect by carrier injection has an effect of decreasing the refractive index, while heat produced by carrier injection has an effect of increasing the refractive index. For this reason, the refractive index is decreased with an increase in carrier injection amount, i.e., an increase in injection current, while the refractive index is increased due to heat produced by the current. Therefore, the refractive index is saturated to a predetermined value, and a tuning range of a selected wavelength is limited to a narrow range.